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PyOS_vsnprintf() underflow leads to memory corruption #46840
Description
| BPO | 2588 |
|---|---|
| Nosy | @gpshead, @amauryfa, @abalkin |
Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.
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GitHub fields:
assignee = 'https://github.com/gpshead'
closed_at = <Date 2008-06-02.00:08:34.074>
created_at = <Date 2008-04-08.15:55:09.533>
labels = ['type-security', 'interpreter-core']
title = 'PyOS_vsnprintf() underflow leads to memory corruption'
updated_at = <Date 2009-04-14.13:54:38.391>
user = 'https://bugs.python.org/jnferguson'bugs.python.org fields:
activity = <Date 2009-04-14.13:54:38.391>
actor = 'psss'
assignee = 'gregory.p.smith'
closed = True
closed_date = <Date 2008-06-02.00:08:34.074>
closer = 'gregory.p.smith'
components = ['Interpreter Core']
creation = <Date 2008-04-08.15:55:09.533>
creator = 'jnferguson'
dependencies = []
files = []
hgrepos = []
issue_num = 2588
keywords = ['patch']
message_count = 12.0
messages = ['65174', '65184', '65186', '65226', '65227', '65228', '65242', '65246', '65254', '67399', '67620', '85969']
nosy_count = 5.0
nosy_names = ['gregory.p.smith', 'amaury.forgeotdarc', 'belopolsky', 'jnferguson', 'psss']
pr_nums = []
priority = 'normal'
resolution = 'fixed'
stage = None
status = 'closed'
superseder = None
type = 'security'
url = 'https://bugs.python.org/issue2588'
versions = ['Python 2.5']
Activity
jnferguson commentedon Apr 8, 2008
The PyOS_vsnprintf() contains the caveat that the length parameter
cannot be zero, however this is only enforced via assert() which is
compiled out. As a result if the length parameter is zero then the
function will underflow and write a null byte to invalid memory.
53 int
54 PyOS_vsnprintf(char *str, size_t size, const char *format, va_list va)
55 {
56 int len; /* # bytes written, excluding \0 */
57 #ifndef HAVE_SNPRINTF
58 char *buffer;
59 #endif
60 assert(str != NULL);
61 assert(size > 0);
62 assert(format != NULL);
[...]
65 len = vsnprintf(str, size, format, va);
[...]
91 str[size-1] = '\0';
92 return len;
93 }
amauryfa commentedon Apr 8, 2008
I think that programming errors against the python API are best checked
with asserts: I develop in development mode (with asserts enabled), then
I want my released program to run at full speed.
Other thoughts?
jnferguson commentedon Apr 8, 2008
I can generally agree with that, and I admit I haven't verified all of
the code paths here- theres several hundred of them leading into this
function, are you positive all of them are safe? (seems like it would be
easier to just move the check into an if than sitting down and verifying
that all XXX hundred code paths are safe).
In the other bug, I have verified code paths into it, for instance test
the misallocation poc in 2586 as an example
abalkin commentedon Apr 9, 2008
As long as snprintf is used with a fixed size buffer using an idiom
snprintf(buffer, sizeof(buffer), ..)
there is no issue because sizeof(buffer) cannot be zero. AFAICT, this
is how python uses PyOS_vsnprintf wrapper.
On the other hand, may this is a good opportunity to revisit the
decision to make PyOS_vsnprintf semantics different from C99 vsnprintf.
C99 defines snprintf semantics as follows:
The snprintf() function shall be equivalent to sprintf(), with the
addition of the n argument which states the size of the buffer referred
to by s. If n is zero, nothing shall be written and s may be a null
pointer. Otherwise, output bytes beyond the n-1st shall be discarded
instead of being written to the array, and a null byte is written at the
end of the bytes actually written into the array.
<http://www.opengroup.org/onlinepubs/000095399/functions/printf.html\>
jnferguson commentedon Apr 9, 2008
I do agree with your point about snprintf(..., sizeof(x), ...)-- my
single biggest point (and maybe i'm just not seeing it), is that there
appears to be no good reason for having this caveat and in turn its
essentially just code waiting to break; with as commonly used of a
function as it is, it's really a matter of when and not so much if.
While no one seems to ever use it this way, don't forget that a good
alternative to asprintf() is calling sprintf() with a length of zero to
get the length (in compliant implementations), allocating the memory and
then calling it again.
abalkin commentedon Apr 9, 2008
On Tue, Apr 8, 2008 at 9:21 PM, Justin Ferguson <report@bugs.python.org> wrote:
Remember that PyOS_vsnprintf was introduced back in 2001 when
(according to the comments in the file) not all platforms provided c99
compliant implementations. If you can verify that the situation has
changes for the supported platforms, I think you will have a good case
for making the wrapper c99 compliant.
jnferguson commentedon Apr 9, 2008
Actually, I'm not sure things are any better today- even the same
operating system but different versions have inconsistencies, for
instance on some versions of RHEL the vsnprintf() can fail during
unicode conversion. MSVCRT still returns -1 on truncation, et cetera.
That said, theres plenty of other implementations that manage this
without the potential of underflowing a buffer
abalkin commentedon Apr 9, 2008
On Wed, Apr 9, 2008 at 1:16 PM, Justin Ferguson <report@bugs.python.org> wrote:
..
Do you have in mind something like the following?
===================================================================
I would be +0 on such change.
jnferguson commentedon Apr 9, 2008
Yep, that works for me.
gpshead commentedon May 26, 2008
Fixed in trunk r63734 using Alexander's suggested fix.
I will backport this to release25-maint.
gpshead commentedon Jun 2, 2008
Fixed in release25-maint r63883.
psss commentedon Apr 14, 2009
Justin, is there any reproducer available for this issue?
Thanks!